What Is EHV-1?
Friday, May 29, 2015

by Tuffy Owens

I am by no means an expert on this topic. However, I do have a computer and access to the internet. I’ve also got a phone and the ability to call people much smarter than I am and much better informed to listen to what they have to say. The following is a compilation of information that I have gathered over the past few weeks of dealing with this terrible virus head-on as the manager of the cancelled Madison Classic Horse Show.

The Equine Herpesvirus, EHV, are viruses that are found in most horses all over the world. Almost all horses have been infected with the virus and have no serious side effects. The strain of this virus that we are most familiar with is EHV-1. EHV-1 can cause four manifestations of disease in horses, including neurological form, respiratory disease, abortion and neonatal death. It is unknown what causes some of the horses to develop the serious neurological forms that may be fatal.

EHV-1 myeloencephalopathy (EHM) results from widespread vascular or blood vessel injury after damage to the lining of the blood vessels of the blood brain barrier. Neurologic signs result from inflammation of the blood vessels, blood clots, and death of neurologic tissue. EHM cases occur singly or can affect multiple exposed horses.

EHV-1 is contagious and spread by direct horse-to-horse contact via the respiratory tract through nasal secretions. It is important to know that this disease can also be spread indirectly through contact with physical objects contaminated with the virus:

The air around the horse that is shedding the virus can also be contaminated with infectious virus. Although it is known that the virus can be airborne, it is difficult to establish the distance the virus can spread in this manner under typical horse management and environmental conditions. The virus is estimated to be viable for up to 7 days in the environment outside of the horse’s body. After an EHV infection, incubation period may be as short as 24 hours, but is typically 4-6 days, and can be longer. EHV-1 typically causes a two-phase fever peaking on day 1 or 2 and again on day 6 or 7.

All of the proceeding information has been taken from the American Association of Equine Practitioners website and can easily be found there if you want to continue this as an independent study.

So those are the facts of the disease itself. Now, how do we go about keeping our horses safe? Biosecurity is a huge part of it. Most important is to first clean equipment and horse housing areas. The virus is easily killed in the environment by most disinfectants. Conventional disinfectants and detergents are the best. It is important to perform hand hygiene (wash hands with soap and dry thoroughly or use alcohol-based hand sanitizer) when moving between horses that are grouped separately to avoid spreading pathogens that may contaminate your hands.

Beyond simple biosecurity measures we come to the point of greatest contention, vaccination. All available vaccines make no label claim to prevent the myeloencephalitic form of EHV-1 (EHM). That sounds bad. However, vaccines may assist in limiting the spread of outbreaks of EHM by limiting nasal shedding EHV-1 and dissemination of infection.

Basically, the idea in layman’s terms is that the vaccinations available are not going to be a 100% protection against EHM. What the vaccination does provide is a boost to the horse’s immune system and a better ability to fight the virus. A huge benefit is the limiting of nasal shedding, which is the way the virus is transmitted. Less shedding equals less transfer. That is easy math right there.

An Oklahoma State University study dealing directly with a neuropathogenic EHV-1 strain found that the viral load in neurologic tissues was lower in vaccinated mares than in control mares; and that the vaccinated mares' viremia (virus circulating in the bloodstream) levels were lower than those found in control mares. The control mares also exhibited more severe neurologic signs compared to vaccinated mares.

The University of California at Davis conducted a study in 2007 that found that while there is no reliable method of preventing EHM, implementation of routine management practices aimed at reducing the likelihood of introducing and disseminating EHV-1 infection is justified. However they did find that “whereas there is good evidence that inactivated EHV-1 vaccines that stimulate high levels of virus neutralizing antibody are capable of reducing viral shedding and the incidence and severity of respiratory disease in both ‘field’ and experimental settings, compelling evidence for their efficacy in preventing EHM is lacking.” The UC Davis study went on to test a modified live vaccine called Rhinomune MLV with interesting results. Five horses challenged with the neuropathogenic strain of EHV-1 two weeks after completion of a two-dose immunization series with Rhinomune shed significantly less virus for a substantially shorter period than did a group of five unvaccinated control horses. In addition, none of the five Rhinomune-vaccinated horses developed neurological signs after challenge, whereas three out of five horses in each of the other two groups did.

There is no “right” answer here, I’m sorry to say. The study sizes have been too small to definitively say that vaccinating against EHV-1 is the best course of action for your horse, but the data at least leads one in that direction.

We as a show horse industry are behind the times with regards to fighting this disease. This was pulled directly from the Stall Application page of the Keeneland website under their list of requirements:

Record of vaccination against Equine Herpes Virus Type 1 (EHV-1) by a licensed veterinarian a minimum of fourteen (14) days, and no more than one hundred twenty (120) days of entry into our stable area(s). Once a horse receives the initial EHV-1 vaccination it only needs to be re-vaccinated every one hundred twenty (120) days, hence if you vaccinated your horse last January then vaccinate him/her again 120 days later. The minimum fourteen (14) day vaccine requirement applies to horses that had NOT been vaccinated for EHV-1 within the past one hundred twenty (120) days. The Certificate of Veterinary Inspection (CVI) shall indicate the date of the vaccination, the specific vaccine product, vaccine lot number, and vaccine expiration date, used to vaccinate each horse.

Here is a requirement from the United States Equestrian Team:

All horses have received regular and consistent vaccinations for Equine EHV-1 & 4 (either modified live or killed vaccine) no sooner than 7 days prior and with most recent booster being within 90 days prior to entering the USET Foundation. Most EHV-1 & 4 vaccines are only considered effective for 90 days. Check with your veterinarian about which vaccine you should use.

Every racetrack in Kentucky requires proof of vaccination against EHV-1. They have required this since 2007. Since 2007, they have had no reported cases of EHM at a Kentucky racetrack. Now that isn’t a scientific study by any means, and they have also increased their biosecurity awareness, but that should give you a good idea of the effect of their program.

The Kentucky State Veterinarian’s office has this to say on the matter:
We acknowledge the available vaccines’ labels make no claim to prevent neurologic disease; but based on our experience managing outbreaks of this disease, and in consultation with infectious disease experts and research scientist, we continue to be of the opinion the vaccine does have a meaningful level of efficacy and may aide in reducing the impact of a disease incident.

That same statement from the Kentucky State Veterinarian’s office also included this nugget of information: United States Equestrian Federation (USEF) amended their vaccination rule earlier this year and will begin requiring EHV-1 vaccination of horses participating at USEF sanctioned events. The rule, requiring vaccination at 6mo intervals, has been formally adopted and though it doesn’t go into effect until 12/1/15, my understanding is many USEF event managers have proactively included the vaccination as a requirement for participation in USEF events being held this summer.

Rather than rush that information out to the masses, Saddle Horse Report waited for independent confirmation of this amendment before publishing. It has been confirmed to us that this is, in fact, a reality. Horses attending USEF sanctioned events will be required to show proof of vaccination against EHV-1 in 2016.

It makes sense. It’s time. Hopefully the non-rated shows will follow suit. Perhaps the UPHA will follow the USEF’s lead and recommend this same protocol for all of their Chapter Shows?

For years we have sat and watched as EHV-1 killed horses around the world and thought, "That's just those 'other breeds.'" So we did nothing to try to get ahead of it as an industry. We made no changes to be proactive. Now it's on our doorstep, and we are forced to react quickly. It's not just those "other breeds" anymore. Reactive instead of proactive again. Hopefully we learn our lesson this time.

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